First, I wouldn't use galvanized anything in a remodel, so I would prefer a brass nipple to connect the spout. But, it's not like there's water in that tub spout all day. Eventually, it will rust, though; brass or copper won't.
Take the showerhead off and see how much flow comes out of the arm. If the flow is good, you know the showerhead is clogged up. If it isn't, it's in the piping or valve.